Histochemical and ultrastructural characterization of the gills in tiger shark, Galeocerdo cuvier (Péron & Lesueur, 1822): Structural strategies for functional adaptation.

The tiger shark (Galeocerdo cuvier) is a dominant marine elasmobranch predator whose respiratory physiology and gill structure enable the oxygen uptake necessary for its significant ecological role in sustaining balance within oceanic environments. The present study investigated the histological, histochemical, and ultrastructural characteristics of gill tissue in the tiger shark to elucidate the morphological adaptations that enable efficient respiration in marine environments. Histologically, the gills were housed in antero-posteriorly compressed gill pouches, with the first pair being hemibranchs and the remaining pairs being holobranchs. The respiratory surfaces consisted of cartilage-supported gill arches bearing numerous filaments with secondary lamellae lined by thin epithelium comprising pavement cells, chloride cells, and sparse goblet cells. Semithin section analysis identified undifferentiated basal cells, lymphocytes, pillar cells, and nucleated erythrocytes within the lamellar structure. Notably, rodlet cells at various maturation stages and intraepithelial telocytes with characteristic telopods were observed throughout the gill epithelium and septum. The well-developed interbranchial septum, a distinguishing feature of elasmobranch gills, exhibited stratified epithelium with diverse cell types and extensive telocyte networks. Histochemical staining demonstrated the presence of polysaccharides and neutral mucins (PAS-positive) in the basement membranes and goblet cells, weak alcian blue staining for acid mucin, abundant collagen fibers in supporting structures, as shown by Masson's trichrome, and elastic fibers in the pharyngeal cavity, gill arches, and blood vessel walls as investigated by orcein stain. SEM revealed well-developed primary lamellae bearing comb-like secondary lamellae, a granular epithelial surface featuring mucous goblet cells and chloride cell projections. The integument overlying the gill region showed densely packed placoid scales (dermal denticles). In conclusion, these findings highlight the complex cellular organization and structural specializations of tiger shark gills that facilitate efficient gas exchange and osmoregulation. Our study sheds light on the adaptive strategies that enable tiger sharks to thrive in their marine environment, providing important new insights into the relationship between gill anatomy and ecological function.